The need to manipulate defined neuronal populations in intact neural circuits in the living experimental animal is urgent for the study of brain disorders, specifically those which are likely caused by abnormal circuitry, such as many of the classical psychiatric disorders. The long-term goal is to develop and refine ever more sophisticated methods for in vivo neuronal manipulation. The overall objective of this particular application is to create a set of new light-driven technologies for studying neural circuit dynamics in the intact brain. These technologies are based on combining optogenetics with bioluminescence, i.e. combining light-sensing molecules (opsins) with biologically produced light through luciferases. The concept of having light-production and light-sensing each genetically encoded permits a) to expand the use of opsins from standard optogenetic applications to also include chemical genetic activation; b) functional testing of synaptic information flow through pre- and postsynaptic targeting of the light-producing and the light-sensing proteins, respectively; and c) translating neural activity via calcium-sensitive luciferases into light whichin turn signals to a light-sensing protein. The rationale for the proposed research is that bimodal interrogation of circuits, functionally mapping long-range and local connectivity, and enabling genetically targeted non-invasive self-regulation of neurons will increase our understanding of neuronal information flow, potentially resulting in new and improved approaches to treatment of neuropsychiatric disorders. Based on strong preliminary data, the objectives will be achieved by pursuing three specific aims: 1) Identify the optimal luciferase-opsin combinations for activating and inhibiting neuronal activity; 2) Express the light-producing luciferase and the light-sensing opsin in cells across synaptic partners; and 3) Combine a luciferase which produces light upon neuronal activation with a light-sensing proton pump. Under the first aim, already working versions of neuronal activators will be optimized and extended to neuronal silencing. Under the second and third aims two novel concepts will be explored, specifically transsynaptic light activation and neuronal activity-controlled light activation. The approach is innovative in that it uses a genetically encoded light source to activate a genetically encoded light transducing molecule. The proposed research is significant, because it enables to ask longstanding questions which currently cannot be addressed with available technology and thus is expected to advance and expand understanding of neuronal circuit function and dysfunction. Ultimately, such increased understanding has the potential to inform new therapeutics that will help reduce the growing mental health problems in the United States.